Ultrasonic, flow measuring devices are applied widely in process and automation technology. They permit easy determination of volume flow and/or mass flow in a pipeline.
Known ultrasonic, flow measuring devices frequently work according to the travel-time difference principle. In the travel-time difference principle, the different travel times of ultrasonic waves, especially ultrasonic pulses, i.e. so-called bursts, are evaluated as a function of the direction the waves travel in the flowing liquid. To this end, ultrasonic pulses are sent at a certain angle to the tube axis both with, as well as also counter to, the flow. From the travel-time difference, the flow velocity, and therewith, in the case of known diameter of the pipeline section, the volume flow, can be determined.
The ultrasonic waves are produced, respectively received, with the assistance of so-called ultrasonic transducers. To this end, ultrasonic transducers are placed securely in the tube wall of the relevant pipeline section. There are also clamp on, ultrasonic, flow measuring systems. In such case, the ultrasonic transducers are pressed externally on the wall of the measuring tube. A great advantage of clamp on, ultrasonic, flow measuring systems is that they do not contact the measured medium and can be placed on an already existing pipeline.
The ultrasonic transducers are normally composed of an electromechanical transducer element, e.g. a piezoelectric element, and a coupling layer. In the electromechanical transducer element, the ultrasonic waves are produced as acoustic signals and led via the coupling layer to the pipe wall and from there into the liquid in the case of clamp-on-systems, and, in the case of inline systems, via the coupling layer into the measured medium. In such case, the coupling layer is sometimes called a membrane.
Between the piezoelectric element and the coupling layer, another coupling layer can be arranged, a so called adapting, or matching, layer. The adapting, or matching, layer performs, in such case, the function of transmitting the ultrasonic signal and simultaneously reducing reflection at interfaces between two materials caused by different acoustic impedances.
Both in the case of clamp-on-systems, as well as also in the case of inline systems, the ultrasonic transducers are arranged on the measuring tube in a shared plane, either on oppositely lying sides of the measuring tube, in which case the acoustic signal, projected onto a tube cross section, passes once along a secant through the measuring tube, or on the same side of the measuring tube, in which case the acoustic signal is reflected on the oppositely lying side of the measuring tube, whereby the acoustic signal traverses the measuring tube twice along the secant projected on the cross section through the measuring tube. U.S. Pat. No. 4,103,551 and U.S. Pat. No. 4,610,167 show ultrasonic, flow measuring devices with reflections on reflection surfaces provided therefor in the measuring tube. Also known are multipath systems, which have a number of ultrasonic transducer pairs, which, in each case, form a signal path, along which the acoustic signals pass through the measuring tube. The respective signal paths and the associated ultrasonic transducers lie, in such case, in mutually parallel planes parallel to the measuring tube axis. U.S. Pat. No. 4,024,760 and U.S. Pat. No. 7,706,986 show such multipath systems by way of example. An advantage of multipath systems is that they can measure the profile of the flow of the measured medium in the measuring tube at a plurality of locations and thereby provide highly accurate, measured values for the flow. This is achieved based on, among other things, the fact that the individual travel times along the different signal paths are weighted differently. Disadvantageous in the case of multipath systems is, however, their manufacturing costs, since several ultrasonic transducers and, in given cases, a complex evaluating electronics need to be used.
There are different approaches for weighting the signal paths. The paper “Comparsion of integration methods for multipath accoustic discharge measurements” by T. Tresch, T. Staubli and P. Gruber in the handout for 6th International Conference on Innovation in Hydraulic Efficiency Measurements, 30 Jul.-1 Aug. 2006 in Portland, Oreg., USA, compares established methods for weighting the travel times along different signal paths for calculating the flow.
WO 1995012110 A1 discloses an ultrasonic, flow measuring device having a measuring tube with planar walls and a straight measuring tube axis and at least one reflection surface in the measuring tube, wherein a normal to this reflection surface has three components different from zero in a right angled coordinate system, whose one axis corresponds to the measuring tube axis. This document teaches that an ultrasonic signal of predetermined width, which is markedly greater than a point shaped signal, has a Gauss shaped sensitivity across this width. This signal is used for flow measurement. The width of the signal corresponds, in such case, approximately to the width of the rectangular measuring tube. If such a signal would pass through the measuring tube parallel to the side walls, the region with the highest sensitivity would extend through the center region of the measuring tube, and, thus, also record the higher flow velocities with higher values. In the case of very small flow velocities, this would lead to a measurement error. The document teaches consequently, further, to irradiate the measuring tube largely homogeneously by leading the ultrasonic signals through all regions of the measuring tube by means of directed reflections. For illustration, the broad ultrasonic signal was represented by individual beam portions. The path lengths of the individual beam portions are equally long, so that the beam portions do not cancel by interference.